On this blouse, it looks like there are two ruffled pieces of fabric that are both sewn to the neckline. Is it possible to sew two edges to one edge? Or would I have to add in an internal line underneath the collar?

When you have these types of questions, I think the MD Technical/Software Questions category is best. I may move this to that location later.

To answer your question, Yes, you can sew multiple patterns together at their edges. On the 2D window side of the program you'll find all your sewing tools. There is a tool called Segment Sewing and you'll see a little notch on the button on the right bottom corner. That means there are other tools hidden behind.

Click and hold on the Segment Sewing tool and it will drop down exposing the M:N Segment Sewing tool. That's the tool you want to use. You can sew M (1 or more) edges to N (1 or more) edges. Once you select the tool, it will give you instructions right on the screen how to use it.

The Segment sewing tools sew complete edges to other complete edges (lines between two points). If you need to sew only a partial edge, use the M:N Free Sewing tool.

The answer to *that* problem is also --yes....you can certainly have more than one layer of ruffles (or any cloth pattern) attached to the same length of seamline.
Just sew on one set of ruffles, let it simulate and settle, then you can either copy/paste/clone that same ruffle and sew it to the same line, or create a different (different appearing) ruffle and sew it on.